This invention relates to disc valves of the type which have a cylindrical housing through which a gaseous or liquid fluid medium flows. A disc is mounted to the housing for pivotal movements about an axis that is transverse to the flow direction through the housing. In a closed position the disc traverses the cross-section of the housing, and cooperating sealing surfaces on the disc and the housing establish a seal between an upstream and a downstream side of the housing. In the open position the disc lies in a plane that is substantially parallel to the fluid flow direction through the housing. A suitable driving mechanism is also provided on such valves for pivotally moving the disc between its closed and open positions. Disc valves of this kind are available in a variety of configurations. Common to all such valves is that when the valve disc is open, and therefore parallel to the flow direction, a seal ring or sealing surface on the side of the disc (relative to its pivot axis) facing in an upstream direction is subjected to substantial wear and tear. The sealing surfaces are particularly stressed when abrasive particles are suspended in a hot fluid medium, since they can damage the sensitive sealing surface on the disc.